1. Comment out the two lines in core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorageSchem‌​a.php that throw exceptions with the text "The SQL storage cannot change the schema for an existing field". About lines 1312 and 1403.
2. In your browser, navigate to /admin/config/development/configuration/single/export. The configuration type is "Field Storage", and choose the field in question. Copy the configuration.
3. Navigate to /admin/config/development/configuration/single/import. . The configuration type is "Field Storage". Paste the configuration. Change the length.
4. Submit the form.

D8

1. Comment out the two lines in core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorageSchem‌​a.php that throw exceptions with the text "The SQL storage cannot change the schema for an existing field". About lines 1312 and 1403.
2. In your browser, navigate to /admin/config/development/configuration/single/export. The configuration type is "Field Storage", and choose the field in question. Copy the configuration.
3. Navigate to /admin/config/development/configuration/single/import. . The configuration type is "Field Storage". Paste the configuration. Change the length.
4. Submit the form.

Note (as you probably already know), if you shorten the length, then existing data will be truncated.